Field of the invention
This invention relates to flexible wiring members such as flexible printed circuits (hereinafter called "FPCs") having excellent flex performance with ease of fabrication.
Printed wiring boards based on organic substrate are roughly divided into two types; rigid type with a rigid, copper-clad laminate consisting of glass-epoxy and paper-phenol substrates and flexible type with a flexible, copper-clad laminate consisting of polyimide and polyester substrates. Copper foil is mainly employed as conductive material for the printed wiring boards. The foil products are classified into electrodeposited and rolled foils depending on the manufacturing processes used.
Of the printed wiring boards, those for flexible printed circuits (FPCs) are fabricated by laminating a copper foil to a resin substrate and joining the layers with adhesive or with the application of heat and pressure into an integral board. In recent years, multilayer boards known as built-up boards have come into extensive use as effective means for high-density packaging or mounting. The copper foil that is used to form components for FPCs is, for the most part, rolled copper foil.
FPCs are largely used in printer heads, hard disk drives, and other components where wiring or conductive connections to movable parts are required. They are subjected to more than a million times of repetitive bending in service. With the recent tendency toward miniaturization and higher performance levels of devices, the requirement for the flex performance is becoming severer than heretofore.
The material for copper foil to be used in FPCs is mostly tough-pitch copper (containing 100-500 ppm oxygen). The foil is manufactured by hot rolling an ingot of such material and then repeating cold rolling and annealing alternately until a predetermined thickness is achieved. The rolled copper foil is then plated for surface roughing for enhanced adhesion to a resin substrate. Following the roughing plating, the copper foil is cut into pieces and each piece is laminated to a resin substrate. To join the copper foil and resin together, an adhesive of thermosetting resin, e.g., epoxy, is used. The adhesive is hardened by heating at 130 to 170.degree. C. for several hours to several days. Thereafter the copper foil is etched to form various wiring or conductive patterns.
The flex property of a copper foil is markedly improved by recrystallization annealing over that of the foil as rolled. Therefore, the foil is used in the annealed state as an FPC component. The annealing is done either by heat treatment after the roughing plating and cutting into a size or by utilizing the heating at the time of joining to the resin substrate. The reason for which the annealing is performed during the course of fabrication rather than using an annealed copper foil from the beginning is that, when the copper foil is soft after annealing, it can be deformed or wrinkled upon cutting and laminating to the resin substrate, and a foil hard as rolled is preferred because of the ease of fabrication into an FPC.
For enhanced flex performance of an FPC, improving the flex fatigue property of a rolled copper foil as the starting material is beneficial. The flex fatigue property of an annealed copper foil is improved with the development of its cube texture. In order to help develop the cube texture, it is effective in the copper foil manufacturing process to increase the final rolling reduction ratio and decrease the grain diameter with the annealing immediately before the final rolling (Japanese Patent Application No. 10-101858).
Actually, however, a copper foil manufactured by such a process shows a sharp drop of the softening temperature due to an increase in the plastic strain accumulated by rolling. In extreme cases the foil, even stored at room temperature, can soften after a long period of storage (refer, e.g., to Japanese Patent Application Kokai No. 10-230303).
As noted already, a softened copper foil, if used in the fabrication of an FPC, can cause troubles such as foil deformation and seriously affect the ease of FPC fabrication. For these reasons it is necessary, when the above manufacturing process is adopted to obtain a copper foil with improved flex property, to heighten the softening temperature of the copper foil to a proper level.